You've been working on a project for days. The paper has been circulated to the relevant folks and the research digested and described.
A colleague who has nothing to do with the project and no expertise whatsoever in the subject matter takes a gander over your shoulder, points to a paragraph, and declares, "That'll never work" and gives some ridiculous reason why.
Your irritation evaporates after ten seconds of thought when you realize the unsolicited opinion has more than some merit; it has a huge amount of merit.
What the meddler was able to do, of course, was to approach the proposal with fresh eyes and no agenda. We fall in love with our projects and rush to dismiss their failings. No matter how many genuises we manage to get on-board, a bad idea is still a bad idea. Some very bright people were behind the creation of the Maginot Line, the Titanic, and, for that matter, the Edsel.
Injecting a review by disinterested parties can at least turn up the lights and end the romance.